The present invention relates to a thermostat; and more particularly, it relates to a thermostat which includes heat anticipation.
It is well known that if a room thermostat were to open its contacts only when the mechanical setting (i.e., the temperature at which the thermostat is set) is reached, the actual temperature of the room would rise well above the mechanical setting and even cause the room to become uncomfortably warm. The reason for this is that when the room temperature reaches the mechanical setting, there is still considerable heat stored in the furnace and duct work, and the blower will continue to deliver this heat to the room after the fuel is shut off. The problem of room temperature overshoot would be even further accentuated because of thermal delays between the ambient temperature of the room being heated and the temperature-sensing element in the thermostat. In other words, the actual temperature-sensing element would reach the mechanical setting long after the ambient temperature of the room reached the mechanical setting. Thus, this delay would cause the room temperature to overshoot even further than that which would be caused only by heat storage in the heating system.
This problem is obviated in a conventional thermostat using a bimetallic temperature sensing element by incorporating a small heater (called the "heat anticipation resistor") adjacent the bimetallic element which generates heat inside the thermostat casing which is conducted directly to the bimetallic element, causing it to switch at the mechanical setting sooner than it otherwise would have switched.
The maximum heat rise of the sensor element due to anticipation heat alone is referred to as the "droop". It is normally of the order of 4.degree. to 5.degree. F. That is to say, if room temperature is at 70.degree. F. and the heat anticipation resistor is continuously energized, but the furnace is not turned on, the temperature of the sensing element will rise approximately four degrees.
The sensing element normally closes at a slightly lower temperature than the temperature at which it opens. The two trip points thus define a hysteresis effect.
In a conventional thermostat of this type, with the trip points centered on the total rise due to anticipation heat alone, the thermostat will have a duty cycle of approximately fifty percent. That is, the contacts will be closed fifty percent of the time and open fifty percent of the time, with no change in room temperature. Further, the thermostat will go through a complete cycle four to six times per hour.
A small change in the room temperature will change the duty cycle of the thermostat contacts over a wide range. That is, for a small decrease in room temperature, the "on" time (when the thermostat contacts are closed) will increase. If the room temperature changes more than one-half of the droop, the thermostat locks on. That is, anticipation heat alone will not cause the contacts to open, the room temperature must also increase.
It would be desirable to have a thermostat with a solid state sensor element as the temperature sensor, doing away with the bimetallic element of current mechanical thermostats. In addition to the low cost and reliability of solid state circuit elements, such a thermostat could be used in controlling the heating or cooling of individual rooms or spaces in a large building, using a central source of heating or cooling, and perhaps incorporating a central computer to facilitate time-of-day programming of temperature for energy conservation. For example, different set points could be stored for different areas in the computer, and changed at different times of the day, taking into account the position of the sun or other environmental factors. However, a simple on/off type of electrical sensor would not have the desirable characteristics of a mechanical thermostat equipped with heat anticipation, as described above.
The embodiments of the present invention are suitable for such systems as well as for direct replacement of conventional mechanical thermostats.
In one embodiment, the sensing element is connected in a bridge circuit, and a signal is generated which is inversely related to temperature. A second signal is generated representative of the set point. These two signals are summed in a comparator circuit. An integrator responsive to the output of the comparator circuit feeds a third signal to be summed with the temperature and set point signals. A fourth signal provides positive feedback to the comparator as a latching signal at the time of output signal change so that once the comparator switches, it remains stable at least until one or the other inputs changes. If the algebraic sum of the signals is greater than a reference voltage (which may be zero volts), the comparator generates an ON signal which may be used to actuate a heat controller. If the algebraic sum of the signals is less than zero, the comparator generates an OFF signal for turning off the heat controller. In operation, if the set point is increased to switch the comparator circuit, the integrator will be initialized to a zero output signal and feed back to the summing circuit a signal having a polarity of opposite sense to that of the set point signal. This integrator feedback signal will increase in magnitude according to a predetermined time constant defined by the integrator. If the set point has been raised only slightly, eventually the output of the integrator will overcome the increase in the set point signal and latching signal, and the output of the comparator will switch to its complementary state, thereby shutting the furnace off. The same effect would have been had if the temperature in the room had decreased to the point where the summed signals would have caused the comparator to change states.
In a second embodiment, the temperature sensing element and the set point signal generator (which may be a variable resistor) are connected in the same bridge circuit, but the overall system operation is the same. The latching comparison function may also be performed mechanically, as with a balance relay.
The present invention thus provides a thermostat capable of using solid state or semiconductor temperature sensors, yet which exhibits the desirable heat anticipation features of present mechanical thermostats.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.